The present invention relates to a sound field control system wherein a monophonic audio signal is converted into stereo-simulated signals.
Spacial sound impression which a listener feels depends on auditory sensations of the ears. When sounds of the same amplitude reach both ears at the same phase, the listener feels as through the sound is coming from the center in front of him, lacking in lateral expanse. On the other hand, when complex sounds of the same amplitude at a various phases are heard, a lateral expanse is sensed.
In the case of steady noise such as white noise and pink noise, the extent of the lateral expanse can be expressed using as a factor only an interaural correlation coefficient .phi.xy(.tau.) of sound heard by both ears. Namely, ##EQU1## wherein, x(t) and y(t) are audio signals reproduced from the right and left loudspeakers, respectively. The value .phi.xy(.tau.) when .tau. is zero represents the correlation coefficient.
However, such a simple physical value is not sufficient to express the lateral expanse felt when a musical sound including a large quantity of impulsive components is heard. Moreover, a feeling of lateral expanse differs in the case of musical sound with transient or impulse sound and in the case of steady noises although the value of the correlation coefficient may be the same.
This is due to the fact that, although there exist reflected sounds from various directions, the human ear is able to discern the direction from which came a sound that first reached the ear, that is, a direct sound of a sound source. More particularly, the human auditory system operates to render the direction from which the initial reflected sound following the direct sound obscure, and to compensate the volume of the direct sound by the reflected sound. Such a characteristics of the auditory system is an important factor in quantitatively expressing the sense of expanse of the musical sounds.
In order to achieve such a sense of lateral expanse, there has been proposed a sound field generating systems such as a surround, presence stereo and omni-sound system for creating the sound field. Each of these systems uses a two-channel audio signal as a sound source. The audio signal is processed so that a component expressing a sense of sound field is effectively strengthened.
Furthermore, there is proposed a sound field control (SFC) system wherein acoustic conditions are added to the two-channel audio signal so as to simulate the effects caused in various reproducing locations. For example, the audio signal is processed by a DASP based on data on sound field collected by way of a proximity four point microphone recording system in famous concert halls of the world, or on data simulated by a computer. The sound reproduced from the processed audio signal is emitted from four speakers, thereby giving the listener a feeling as though he is actually in one of these halls.
Japanese Patent Application Laid Open No. 6-269098 discloses such a SFC system as shown in FIG. 4. Referring to FIG. 4, a monophonic audio signal S(t) is fed to a first SFC processing circuit 10 and a second SFC processing circuit 20. The first and second SFC processing circuits 10 and 20 process the signal S(t) in a different manner so that stereo-simulated signals S.sub.1 (t) and S.sub.2 (t) having a small correlation coefficient therebetween are generated. The stereo-simulated signals S.sub.1 (t) and S.sub.2 (t) are fed to loudspeakers 12 and 22 through respective amplifiers 11 and 21 so as to be reproduced. Namely, in the SFC system, the signals are controlled so as to set the transient interaural correlation coefficient at an optimum value to provide a sense of lateral expanse.
More particularly, FIG. 5 shows the first and second SFC processing circuit 10 and 20 in detail. The SFC processing circuit 10 comprises a left delay element 11 having a plurality of output terminals LO.sub.1 to LO.sub.n so that a plurality of delay times are provided. Similarly, the SFC processing circuit 20 comprises a right delay element 11R having a plurality of terminals RO.sub.1 to RO.sub.n. The delay time becomes longer as the distance between each output terminal and the corresponding input terminal Lch IN or Rch IN becomes longer.
Output terminals LO.sub.1 and LO.sub.2 of the delay element 11 and an output terminal RO.sub.5 of the delay element 11 are connected to an adder 4 so as to generate a first left channel reverberation signal. Output terminals LO.sub.i and LO.sub.i+1 of the delay element 11 and output terminals RO.sub.1 and RO.sub.2 are connected to an adder 5 so as to generate a first right channel reverberation signal. Similarly, output terminals LO.sub.k and LO.sub.k+1 and RO.sub.t and RO.sub.t+1 are connected to an adder 6 to generate a second left channel reverberation signal. Output terminals LO.sub.j, RO.sub.u and RO.sub.u+1 are connected to an adder 7 to generate a second right channel reverberation signal.
The first reverberation signals from the adders 4 and 5 have a relatively small delay while the second reverberation signals from the adders 6 and 7 have a large delay.
The first left and right channel reverberation signals from the adders 4 and 5, respectively, are fed to a first function of correlation control filter 3, and second left and right channel reverberation signals from the adders 6 and 7, respectively, are fed to a second function of correlation control filter 2.
The first right and left reverberation signals with a smaller delay are controlled to have a predetermined interaural correlation coefficient and the second reverberation signal with a large delay are controlled to have a correlation coefficient corresponding to the delay, thereby to provide an appropriate sense of expanse.
The principle of the above-described conventional system is based on a transient interaural correlation coefficient. The filters 2 and 3 control the interaural correlation coefficient to coincide with that of a concert hall said to have excellent acoustics, so that a similar acoustic effect is obtained in an ordinary room.
The correlation coefficient control filters 2 and 3 control the signals by SFC processing and adding a negative-phase sequence component. However, the frequency response in accordance with the correlation coefficient is not considered, so that the sense of expansion is not sufficient.